Heat treatment is used for many industrial processes. It is of high importance, in particular, in the semiconductor technology, such as when manufacturing photovoltaic cells.
When manufacturing solar cells, semiconductor wafers generally undergo three metallization processes: the metallization process for the bus bar, the aluminum rear side contact and the front side contact. Metallization processes are used for contacting the semiconductor wafer and contacts are produced to the doped regions of the semiconductor wafer.
The contacting process generally has three stages: In a first step, a metal-containing paste in the shape of the contact to be produced is applied onto the surface to be contacted, for example, using a printing method such as screen printing. In this manner, the contact can be formed as a contact grid or applied over full-surface onto the semiconductor wafer surface to be contacted. By adding organic solvents and binders being admixed to the pastes, it is thereby ensured that the contact pastes are viscous, at least during the application procedure.
In a second step, the paste applied to the semiconductor wafer is dried at temperatures of about 250° C. so that the solvent and binder contained in the paste evaporates.
In a third step, the semi-conductor wafers to be contacted are exposed to temperatures of 200 to 400° C. during which the remaining organic substances contained in the paste burn away without residue. This is followed by short sintering (of a few seconds) of the paste at temperatures of 600 to 1000° C. The actual formation of the contact between the semi-conductor wafer and the metal paste is effected in this third step and electric conductivity of the contacts is formed during the sintering process.
It is to be observed when contacting semiconductor wafers, firstly, that the contact resistance between the semiconductor wafer and the contact should be low and, secondly, as in particular for solar cells, the smallest possible portion of the effective functional surface of the semiconductor wafer is covered by the contacts. This is especially true when manufacturing contacts for solar cells on the side facing the incident light, as these contacts shadow the semiconductor material due to reflection or absorption of the incident light.
The temperature profiles required during metallization are crucial for the solar cell quality and degree of efficiency to be achieved. In addition to the so-called longitudinal profiles reflecting the temperature-time-curve of the thermal process, so-called cross profiles represent the temperature distribution across the width of the thermal device. While the longitudinal profiles usually have a peak temperature distribution which can for Fast-Firing devices reach up to 1000° C. and represent the firing process, transverse profiles should reflect the most homogeneous possible distribution of temperature along the width of the thermal device. Recording a temperature distribution in vicinity around the peak can be of particular interest, or recording a temperature distribution within a measurement volume comprising the peak.
The firing process is usually performed within a continuous furnace having a plurality of heating zones and a cooling section. The semiconductor wafers are transported through the furnace lying on a metal link conveyor. The heating elements are typically arranged above and below the conveyor. For highly productive manufacturing processes, partially multi-lane continuous furnaces are used in drying and firings devices. A typical drying device can be composed of a combination of five infrared zones and one central convection zone. Within the individual heating zones, the temperature can be controlled independently of the other heating zones, whereby a predetermined temperature profile can be set within the device. It should be noted that the conveyor belt co-determines the temperature distribution within a continuous furnace due to its own thermal capacity.
The quality of a semiconductor wafer contacting is determined by the electrical and mechanical contact formation, where at the same time the semiconductor wafer may not be subjected to any damage. This is particularly important when contacting a solar cell front side, for which any damage can cause short circuits and thereby greatly impair the efficiency of the solar cell.
It is therefore of great importance to determine a temperature profile in the longitudinal as well as in the transverse direction of a furnace and/or a device and to adjust and respectively maintain a desired temperature distribution within the furnace or the device, respectively.
A method for temperature-controlled processing of semiconductor wafers is known from publication DE 103 25 602 B3 in which the temperature of a semiconductor wafer is measured contactlessly within a reaction chamber using a pyrometer. The pyrometer is firmly fixed outside the chamber and through an inspection window detects the radiation emitted by one side of the semiconductor wafer. By using this measuring method, only the temperature of one side of the semiconductor wafer can therefore be detected at one location predetermined by the inspection window. However, it is not possible to detect a temperature profile within the reaction chamber along arbitrary measuring points in a manner independent of the semiconductor wafers to be processed.
Document DE 10 2007 020 176 A1 discloses a measuring system for temperature profile measurement in continuous furnaces in which a temperature sensor with an impedance-matched sensor antenna is driven by a conveyor belt through the furnace. There are monitoring antennas located in a continuous furnace monitoring the temperature determined by the sensor and the position of the sensor.
According to another widespread technique, individual measurement wafers are sent through a continuous furnace. Thermocouples and data recording devices are usually used for temperature recording. The thermocouples are either contacted to a silicon wafer acting as a measurement wafer only by a mechanical preload or attached thereto using suitable adhesives. Contacting thermocouples to silicon wafers is commonly associated with problems, since silicon wafers are brittle and very thin (approx. 180 μm). Mechanical fixation by preloading bears the problem that it can easily be released at the high temperatures given in the devices. Adhesives change the thermal capacity at the measuring location and can distort temperature measurement.
For determining a temperature distribution along the width of a silicon wafer, one thermocouple is not sufficient. Three thermocouples are frequently attached onto the silicon wafer and a detected temperature difference between the measuring locations of the individual thermocouples on the silicon wafer represents a measure for the homogeneity of the temperature distribution. Each individual thermocouple attached on the wafer during temperature detection produces an individual temperature profile with different variations in measurement. Calibration, however, is very complex and also difficult. Calibration is in daily practice usually dispensed with, so that the measurement results are subject to relatively large systematic errors.
Furthermore, the number of temperature profiles generated by the thermocouples is limited in that only a number of thermocouples depending on the width of the wafer can be attached on one wafer. Usually one to four thermocouples are attached on a wafer.
Silicon wafer exhibit varying reflection and absorption properties, in particular due to the antireflection coating. Locally differing reflection and absorption properties can significantly affect the measurements of the individual thermocouples.
In determining these temperature distributions, the same measuring wafer is commonly repeatedly placed on the transport system of the device and driven through the device by the transport system in order to obtain reproducible results. As silicon wafers fitted with thermocouples are used as a measurement wafer, the number of repeated measurements is limited by the life span of the silicon wafer. Due to the very high transport speeds of conveyor belts (4 to 6 meters per minute), there is the further risk that the measurement wafers can be easily damaged. It is also difficult to repeat the measurements in a reproducible manner.
However, not only the temperature profiles in devices for the heat treatment of substrates are of interest, but also other measurement data, for example, data representing flow behavior of convection flows or vibrations of the device. The knowledge of convection flows within furnaces is extremely important for setting predetermined temperature profiles, whereas determining vibrations can allow the risk of damage to the substrates due to shocks be detected at an early stage.
It is an object of the invention to provide a device for the heat treatment of substrates which overcomes the drawbacks of prior art when determining measurement data within these devices. It is additionally an object of the present invention to provide an efficient method for recording measurement data in such a device solving the problems of prior art.